Something occured to me while running Alliance raids today.
Have the Alliance that deals the most cumulative damage rewarded with an additional treasure coffer, or they get an upgraded coffer with three peices of loot instead of two.

The DPS of a party is reliant on everyone doing well, you cant screw other people over, lack of healing, poor emnity control, or deaths, will reduce overall damage output, so everyone just has to play their jobs to their best.
It'd be a flat out three-way competition, a team endurance race of sorts.

There's still an element of luck, you can be paired up with poor performers, but it would give more incentive for everyone to try their best, without being so important that it leads to toxicity against poor performers, its only one or two extra peices of loot after all.